Whoa whoa!
He replaced his heart with a Balor?
A WHOLE BALOR?
or just the heart of a balor?
A whole Balor. Who you then had to fight after killing the dragon he was serving as an organ for ...
Yeah it was a tough fight.

Whoa whoa!
He replaced his heart with a Balor?
A WHOLE BALOR?
or just the heart of a balor?
Whoa whoa!
He replaced his heart with a Balor?
A WHOLE BALOR?
or just the heart of a balor?
Whoa whoa!
He replaced his heart with a Balor?
A WHOLE BALOR?
or just the heart of a balor?
The story of all those modules would work pretty well with 4th Edition, but the final location of the Bastion of Unborn Souls would need to be tinkered with, as there is no longer a plane of positive energy (elemental chaos? an astral domain?).